Analytical method for determining active fluoride in acid solution



United States Patent 3 393 980 ANALYTICAL METl-IOl FOR DETERMINING ACTIVE FLUORIDE IN ACID SOLUTION Andy Albert Johnson, Oak Park, and Peter George Kenedi,

This invention relates to an analytical method for determining the active fluoride content in acid solutions. More specifically, it relates to an analytical control method for determining the active fluoride content in solutions containing other ions which have heretofore made it diflicult-toimpossible to make such determinations accurately.

Fluoride ions may be present in a wide variety of solutions: as a component of various pickling baths wherein metals may be treated to remove oxide scales therefrom; in aqueous solutions which may be added to other systems; in a wide variety of baths used for electrodeposition of metals typified by chromium; in various solutions obtained during the analyses of various rodenticides; etc. Analysis of these solutions which contain fluoride ions together with other ions has always been characteristically ditficult; in particular there has heretofore been no satisfactory method for the determination of active fluoride in acid solutions typified by a bath from which chromium may be electrodeposited. One of the factors which makes analysis of fluoride-containing solutions difficult is the presence of other materials which may interfere with ready determination of fluoride ions. As is apparent to those skilled in the art, many other ions interfere with ready determination of fluoride ions as silicofluoride ions. Presence of alkaline earth metals may permit formation of various insoluble fluorides, typified by calcium fluoride, which render analysis difficult.

A standard analytical procedure for determining fluoride may include that of H. H. Willard and O. B. Winter (Anal. Chem. 5, 6 (1933)), wherein the fluoride is converted to hydrofluosilicic acid which may be distilled from the solution over an extended period of time. It is common to titrate the hydrofluosilicic acid distillate with a thorium compound typically thorium nitrate and thereby precipitate the highly insoluble thorium fluoride in the presence of aqueous sodium alizarin sulfonate, as indicator, to determine the amount of fluoride present.

Although this (or similar techniques) may permit attainment of somewhat satisfactory analytical determinations of the total concentration of fluoride, it is apparent that it involves time-consuming operations including distillation and titration, and that it requires special equipment in order to handle successfully the corrosive fluoride which may be present.

As is well known to those skilled in the art, chromium metal may be deposited onto the surface of various base metals by a wide variety of techniques. Commonly baths from which chromium may be electrodeposited may contain chromic acid (Q0 in amount of 50 to 500 g./l. (grams per liter), typically 250 g./l., and sulfate ion in amount of 0.3 to g./l., typically 2.5 g./l.; the ratio of chromic acid to sulfate in the baths may be maintained in the region of 50:1 to 150:1, typically 100:1, when the solution contains about 250 g./l. of chromic acid. Baths of this type will normally also contain addition agents or catalysts therein to permit attainment of desired results.

Typical of the agents which may be present may be complex fluoride ions such as the silicofluoride (SiF ion, provided, for example, by addition to the bath of potassium silicofluoride (K SiF Other complex fluoride ions which may be present during normal operation of the bath may include ions of BF4 1, AlF TH ZrF etc. Typically these ions, e.g., the SiF ion, may be pres- 3,393,980 Patented July 23, 1968 ent in amount sufficient to exceed their solubility. In the case of K SiF the SiE; ions may be present in amount of at least about 0.5 g./l. and commonly 1.5-6 g./l. Maintenance of the concentration of SiF within these limits has heretofore been considered a desirable criterion for proper operation of chromium plating baths.

As is well known to those skilled in the art, maintenance of the desired concentrations of SiF ions has heretofore been effected by insuring presence of this material in amount over and above its solubility limit. This excess of, e.g., potassium silicofluoride is typically present in the bath as a solid phase which may rest on the bottom of the bath.

Other chromium plating systems which may be employed may contain other types of complex fluoride additives typified by so-called simple fluorides such as sodium fluoride, by fluosilicic acid, by sodium silicofluoride, or by magnesium silicofluoride.

In operation of chromium plating baths, particularly those containing a soluble fluoride, typified by sodium fluoride, magnesium silicofluoride, fluosilicic acid, or sodium silicofluoride, it has been found that the proper operation of the bath may in time become diflicult-to-impossible, even though standard analytical procedures indicate that the bath is still apparently a suitable chromium plating bath. Specifically, for example, it may be found that the plate deposited from such a bath may be dull, rough, and nodular. When this has heretofore been found to occur, testing for the fluoride content by various standard analytical tests typically including that proposed by H. H. Willard and O. B. Winter supra may give results which indicate that the total fluoride ion concentration is at a desired level.

' Similarly, if inadvertently the level of SiF ion present be substantially less than that hereinbefore noted, the plate may be similarly defective. Standard analytical techniques for determining concentration of fluoride in the bath may reveal that the fluoride concentration appears to be within the satisfactory range.

Sometimes the SlF ion which may be present, as determined by standard analytical methods, may have been inertized by reaction with or combination with various metal contaminants such as iron, silicon, aluminum, or trivalent chromium and thus may not exhibit catal ytic effect in accordance with its apparent analyzed concentration. Such fluoride may be referred to as inactive fluoride. Active fluoride may be the fluoride (including complex fluoride) which is capable of performing its intended function, e.g., exerting a catalytic effect in chromium plating baths.

It has heretofore been common when conditions have existed in which fluoride may be inactive. to consider the plating bath as spent (although the reasons why it should be spent have not been known) and to normally replace it by a fresh chromium plating bath.

It is an object of this invention to set forth a novel analytical technique for determining the active fluoride content of various acid solutions. Another object of this invention is to set forth a technique for ready determination of active fluoride in chromium plating baths. Other objects of this invention will be apparent to those skilled in the art on inspection of the following description.

In accordance with certain aspects of this invention, the determination of the active fluoride content of an acid solution containing fluoride may by effected by immersing aluminum metal in said acid solution, maintaining said aluminum metal in said solution for a predetermined time during which a portion of said aluminum metal dissolves in porportion to the active fluoride content of said solution, withdrawing said aluminum metal from said solution, determining the weight loss from said aluminum metal during said immersion, and converting said weight loss into values showing the concentration of active fluoride.

It is a feature of this invention that it may be employed in the control of a wide variety of fluoride-containing solutions including chromium plating solutions or baths. Typically these chromium plating baths may contain chromic acid (CrO in amount of 50 to 500 g./l., say 250 g./l.; sulfate ion in amount of 0.3 to g./l., say 1 g./l., and silicofluoride in amount of 0.5 to 10 g./l., say 2.5 g./l. If such a bath with which this inventiion may be employed is a non-self-regulating bath, it may contain from 1 to g./l., say 4 g./l. of fluosilicic acid or 1 to 5 g./l., say 2 g./l. of sodium fluoride.

If the solution or bath with which this invention is to be employed is a self-regulating bath, it may contain in addition to the chromic acid, strontium sulfate (as a source of sulfate ion) in amount suflicient to exceed its solubility, typically in amount of at least about 1 g./l., say 1 to 4 g./l., together with preferably potassium silicofluoride, K SiF (as a source of silicofluoride ion) in amount sufficient to exceed its solubility, typically at least about 0.5 g./l, say 1 to 6 g./l., say 3 g./l. Typically these solutions may, as is well known to those skilled in the art, be used to chromium plate for varying periods of time. During this period of operation under more-orless standard conditions the bath will deposit a bright, lustrous finish of electro-deposited chromium. After extended periods of time and when excessive contamination has occurred, fluoride-containing chromium plating baths (typified by those containing chromic acid, and combinations of catalysts typically including sulfate and fluoride or sulfate and silicofluoride, SiF may be found to give chromium plate which is streaky, dull, nodular, and generally undesirable.

Determination of the active fluoride content of such a chromium plating bath, in accordance with this invention may be effected by preferably isolating an aliquot test portion of the bath. Typically the aliquot portion may be 100-1000 ml., say 500 ml. Preferably the analytical test may be effected at 43 C.il C.; and accordingly the aliquot may be placed within a constant temperature water reservoir maintained at this temperature for at least one hour until water reservoir temperature is reached.

In practice of the invention, a piece of aluminum may be employed. This piece may be pure aluminum or aluminum alloy. Preferably this aluminum may be in the form of a strip or sheet, typically 0.2 to 2, say 1 mm. thick, typically 5 mm. to mm., say 10 mm. wide, and

mm. to 150 mm., say 75 mm. long. The strip may be cleaned typically by wiping it with a cloth and bent into convenient U-shape which facilitates placing the strip in the aliquot and maintaining it in exposed relation to the aliquot with minimum area of contact with the container. The so-treated, cleaned strip may then be weighed to the nearest tenth of a milligram. Satisfactory results may normally be obtained by use of a strip which typically may weigh from 1 g. to 5 g., typically about 2 g.

The so-treated aluminum strip may then be placed in central portion of the container in which the aliquot solution is maintained, so that the strip is standing up in the center of the container. The strip may be maintained therein for an exact predetermined period of time, typically 10 minutes to minutes, and preferably for exactly 30 minutes. During this time, the solution is maintained preferably at the hereinbefore noted temperature of about 43 C. i.e. 43 C.i1 C., and no agitation is provided other than that generated by the reaction of the aluminum with the solution. Reproducibility of the results may be insured by controlling the test to absolutely minimize or eliminate outside agitation.

It is a particular feature of this invention that use of aluminum in this particular process is both unique and unexpected. 'It has been found for example that if other commonly available metals, typified by stcel, brass, cop- 4 per, nickel, zinc, magnesium, or lead be employed, the results are not correlatable with the concentration of active fluoride. This may be because the particular metal may be substantially etched by the chromic acid and other components of the sloution, typically the sulfate ion, or because it is substantially unresponsive to the concentration of active fluoride, or because of other causes. For example, each of the noted metals appears to be substantially unetched by the silicofluoride ion, SiF in a chromic acid solution upon simple immersion.

At the end of the predetermined period of time, typically exactly 30 minutes, the aluminum strip described above may be separated from the solution, and then washed, and dried. The loss in weight in milligrams is determined, and this is converted to milligrams weight loss per square centimeter of surface per hour. This may be done by dividing the weight loss in milligrams by the area in square centimeters and by the number of hours during which the test is conducted.

It will be apparent to those skilled in the art that the specific results obtained in terms of the loss of weight in milligrams per square centimeter per hour may be dependent upon the specific details of the particular procedure employed. Accordingly in order clearly to set forth this invention, reference will hereinafter be made to the following preferred specific procedure which has been found to give reproducible results when used to determine active fluoride in chromium plating baths.

(1) Withdraw from the chromium plating solution to be tested, a desired sample which is representative of the operating bath.

(2) Prepare aluminum strips from aluminum alloy 1100 (99.5% pure aluminum) by cutting a one millimeter thick sheet into 12 mm. x mm. strips. Prior to use in the test procedure, each strip should be wiped to superficial apparent cleanliness with cotton gauze. The cleaned strip is then bent into U-shape and weighed to 0.0001 g.

(3) The aluminum strip is then placed in the aliquot at 43 C.i1 C. so that the strip is standing up on edge in the center of the bottom of the aliquot container.

(4) The strip is maintained for exactly 30 minutes within the quiescent bath without agitation, other than that occurring because of whatever reaction may take place between the strip and the bath.

(5) At this time, the strip is removed, rinsed with distilled water, and dried by immersion in acetone followed by blowing with air.

(6) The loss in weight may be measured and converted to milligrams per square centimeter per hour, and this may be readily correlated with the active fluoride content of the bath by reference to Table I, infra.

A standard for use in connection with this technique may be prepared by using as the aliquot bath, a freshly prepared solution containing 250 g./l. chromic acid, CrO 1.0 g./l. of sulfate (derived from sulfuric acid) and varying concentrations of SiF (derived from H SiF These solutions may be subjected to the above test at 43 C.il C. in a standard series of experiments. The series of experiments indicates that it is possible to directly correlate the concentration of Silfl, ion with the weight loss of the sample, as may be observed from the following table which sets forth results in the normally used levels of SlFg concentration.

TABLE I Conc. g./l. Weight loss SiF mg/ cm. hr. 4 5.20 3 4.65 2 3.54 1 2.34 O 0.04

As will be apparent from inspection of the above table, these values, when plotted, form a smooth, continuous curve whereon intermediate values may be readily interpolated. For example in accordance with their standard technique, if the weight loss of aluminumis found to be, 1.5 mg./cm. /hr., it can then be determined that the bath which was tested has an activity equivalent to a freshly prepared bath containing 0.65 g./l. of SiF It is of particular advantage to be able to determine this value. Attempts to determine the active fluoride content by the hereinbefore noted known prior art techniques are complicated and may incorrectly indicate, in this particular instance, an active fluoride content corresponding to a content of for example 2.0 g./l. SiF Consideration of the bath based upon the results of these prior art analytical techniques would indicate that the bath contained the desired amount of fluoride catalyst and accordingly no reason could be given for the failure of the bath to deposit the desired plate. Furthermore, no readily apparent method for correcting the poor performance was known, and accordingly baths of this type 'were frequently discarded.

The analytical technique of the instant invention, which may distinguish between a total silicofluoride content of 2.0 g./l. and an active fluoride content (which is responsible for obtaining the desired plate) equivalent to an effective silicofluoride content of 0.65 g./l. permits one to readily understand that the bath should be modified and how much is should be modified.

As will be apparent to those skilled in the art, modification may be effected depending upon the particular composition of the bath. In certain baths modification may be effected by addition to the bath of a sufficient amount of e.g. sodium fluoride or other active fluoridecontaining material in amount sufficient to raise the total active fluoride to the desired level which may for example be equivalent to 2.0 g./l. of effective silicofluoride. In the case of certain other baths, the problem may have arisen because of the deficiency of active fluoride, i.e., resulting from substantial contamination by certain metallic cations, and some of the available fluoride may be inactivated by being complexed. The solution may readily be modified to create therein the desired level of active fluoride based upon the level present in the solution and determined by the technique of this invention. It the deficiency is due to complexing of available fluoride by contaminating metals such as iron or aluminum, a practical remedy may be to discard a portion of the contaminated solution and to restore the solution to its original concentration with new material.

Practice of this invention may be more readily understood from inspection of the following illustrative examples:

Example I In this example a chromium plating bath containing and 2.2. g./l. of silicofluoride by standard prior art tests, operated under production hard chromium plating conditions for an extended time, was found to produce deposits which were pitted and dull. The solution contained 22.0 g./l. of total metallic contaminants, mostly Cr+ and 1%. On the basis of standard prior art analytical tests, the solution composition was acceptably normal while de posit appearance indicated low catalyst concentration. The solution was tested by the process of the instant invention wherein a clean, dry, pro-weighed piece of aluminum was used and, at the conclusion of a 30 minute test, was found after rinsing, drying and weighing to have lost 070 mg./cm. /hr. This rate of loss of weight was correlated with the figures in Table I from which it was determined that the active fluoride concentration, expressed as silicofluoride anion, was 0.28 g./l., thus confirming the earlier diagnosis of low catalyst.

The concentration of metallic contaminants was reduced by discarding half of the solution and restoring its volume with water and an appropriate amount of new materials. After thorough stirring, plating tests indicated 6 a bright and smooth deposit and analysis by prior art methods showed .38 g./l. SiFp while a test by the instant invention showed 3.15 mg./cm. /hr. weight loss which corresponds to an active fluoride concentration, expressed as silicofluoride of 1.60 g./l.

Example II In this example a chromium plating bath containing in solution 307 g./l. of chromic acid, 1.77 g./l of sulfate and 5.1 g./l. of silicofluoride by standard prior art methods, operated under production hard chromium plating conditions for an extended time, was found to produce deposits with were pitted and dull. The solution contained 4.8 g./l. of metallic contaminants, mostly Fe+ and Al On the basis of standard prior art analytical tests, the solution composition was acceptably normal while deposit appearance indicated low catalyst concentration.

This solution was tested by the process of the instant invention wherein a clean, dry, pre-weighed piece of aluminum was used and at the conclusion of a 30 minute test, was found, atfer rising, drying and weighing, to have lost 3.6 rng./cm. /hr. This rate of loss of weight was correlated with the figures in Table I from which it was determined that the active fluoride concentration, expressed as silicofluoride anion, was 2.0 g./l., thus confirming the earlier diagnosis of low catalyst.

The concentration of metallic contaminants was reduced by discarding a portion of the solution and restoring it to its original volume with water and the appropriate amounts of new materials. After thorough stirring, plating tests indicated a bright and smooth deposit and analysis by the prior art methods showed 5.4 g./l. of silicofluoride while a test by the process of the instant invention showed 5.1 mg./cm. /hr. weight loss which correspond to an active fluoride concentration, expressed as silicofluoride, of 3.8 g./l.

Example III In this example a chromium plating solution containing 459 g./l. of chromic acid, 1.60 g./l. of sulfate ion and 2.0 g./l. of silicofluoride ion by standard prior art tests, operated under production decorative chromium plating conditions for an extended time, was found to produce deposits which exhibited patches or blotches of dull or milky plate. The solution contained 30 g./l. of trivalent chromium. On the basis of standard prior art analytical tests, the solution composition was within the low catalyst concentration.

This solution was tested by the process of the instant invention wherein a clean, dry, pre-weighed piece of aluminum was used and, at the conslusion of a 30 minute test, was found after rinsing, drying and weighing, to have lost 0.78 mg./cm. /hr. This rate of loss of weight was correlated with the fingers in Table I from which it was determined that the active fluoride concentration, expressed as silicofluoride anion, was 0.3 g./ 1., thus confirming the earlier diagnosis of low catalyst.

The concentration of metallic contaminants was reduced by electrolyzing the solution under conditions which oxidized the trivalent chromium to hexavalent chromium and a repeated plating test gave a bright and normal deposit. Analysis by the prior art methods showed 1.9 g./l. SiFe while a test by the instant invention showed 3.1 mg./cm. /hr. weight loss which corresponds to an active fluoride concentration, expressed as silicofluoride, of 1.52 g./l.

Example IV A pickling bath suitable for use in the pickling of cast iron may be prepared by dissolving 29.6 cc. of commercial 93.29% sulfuric acid and 29.6 cc. of commercial 48% hydrofluoric acid in 3.8 liters of Water. This acid solution may be analyzed by the process of this invention and may be found to contain about 4.0 g./l. of active fluoride.

As many embodiments of this invention may be made Without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modications and variations as come within the scope of the appended claims.

What is claimed is:

1. The process for determining the active fluoride content of an acid solution containing fluoride which comprises immersing aluminum metal in said acid solution, maintaining said aluminum metal in said solution for a predetermined time during which a portion of said aluminum metal dissolves in proportion to the active fluoride content of said solution, withdrawing said aluminum metal from said solution, determining the weight loss from said aluminum metal during said immersion, and converting said weight loss into values showing the content of active fluoride in said solution.

2. The process for determining the active fluoride content of an acid solution as claimed in claim 1 wherein said acid solution contains inactive fluoride.

3. The process for determining the active fluoride content of an acid solution as claimed in claim 1 wherein said acid solution contains silicofluoride.

4. The process for determining the active fluoride content of an acid solution as claimed in claim 1 wherein said acid solution is maintained at a substantially constant temperature during said immersion.

5. The process for determining the active fluoride content of an acid solution as claimed in claim 1 wherein said acid solution is maintained at about 43 C. during said immersion.

6. The process for determining the active fluoride content of an acid solution as claimed in claim 1 wherein said aluminum metal is a strip of aluminum.

7. The process for determining the active fluoride content of an acid chromium plating solution containing chromic acid and fluoride which comprises immersing aluminum metal in said chromium plating solution, maintaining said aluminum metal in said solution for apredetermined time during which a portion of said aluminum metal dissolves in proportion to the active fluoride content of said solution, withdrawing said aluminum metal from said solution, determining the weightloss from said aluminum metal during said immersion, and converting said weight loss into values showing the content of active fluoride ion in said chromium plating solution.

8. The process for determining the active fluoride content of an acid chromium plating solution as claimed in claim 7 wherein the fluoride is silicofluoride.

References Cited UNITED STATES PATENTS 2,593,447 4/1952 Hcsch. i 2,841,540 7/1958 Smith 2388 3,129,148 4/1964 Stenibrecher et a1. 23-;230 X FOREIGN PATENTS 536,712 2/1957 Canada.

OTHER REFERENCES Fischer et al.: Aluminum Fluoride Hydrates, in Chemical Abstracts, vol. 44, 1950, p. 8275h. Article in 2388 Lit.

Willard, H. H., Diehl, H.: Advanced Quantitative Analysis, D. Van Nostrand Co. Inc., New York, 1943, QD 101 W55, 1943, 02. (Copy in Group 171, p. 31 relied on.)

MORRIS O. WOLK, Primary Examiner.

R. M. REESE, Assistant Examiner. 

1. THE PROCESS FOR DETERMINING THE ACTIVE FLUORIDE CONTENT OF AN ACID SOLUTION CONTAINING FLUORIDE WHICH COMPRISES IMMERSING ALUMINUM METAL IN SAID ACID SOLUTION, MAINTAINING SAID ALUMINUM METAL IN SAID SOLUTION FOR A PREDETERMINED TIME DURING WHICH A PORTION OF SAID ALUMINUM METAL DISSOLVES IN PROPORTION TO THE ACTIVE FLUORIDE CONTENT OF SAID SOLUTION, WITH DRAWING SAID ALUMINUM METAL FROM SAID SOLUTION, DETERMINING THE WEIGHT LOSS FROM SAID ALUMINUM METAL DURING SAID IMMERSION, AND CONVERTING SAID WEIGHT LOSS INTO VALUES SHOWING THE CONTENT OF ACTIVE FLUORIDE IN SAID SOLUTION. 